iommu/amd: Check for the right TLP prefix bit

Unfortunatly the PRI spec changed and moved the
TLP-prefix-required bit to a different location. This patch
makes the necessary change in the AMD IOMMU driver.
Regressions are not expected because all hardware
implementing the PRI capability sets this bit to zero
anyway.

Cc: stable@vger.kernel.org # v3.3
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>